This invention relates to an improved apparatus for electrically heating pipes and particularly thawing frozen pipes, hydrants, electric cable ducts and the like.
Such apparatus is known which comprises a movable trailer incorporating a generator, a step-down transformer and two bus bars to which electric cables can be connected. Each bus bar is arranged to accept several cables, one or more cables of one bus bar being connected to one end of a pipe (or pipe system) to be thawed and one or more cables of the other bus bar being connected to the other end of the pipe to be thawed.
One problem with the known apparatus is due to the fact that the operators generally have little knowledge of electrical theory. They connect up one pair of cables and run the machine at a current output of around 200 amps. If, after 10 minutes or so there is no evidence of thawing they simply increase the current as it seems logical that will bring about the desired result. They do not think it necessary to add more cables, which is time-consuming, when one pair of cables evidently can carry easily the increased current. This is because they do not appreciate that the resistance of the cables themselves is an important factor especially where the pipe is copper which has a very low resistance. In this case the I.sup.2 R heat generated in the cables will increase at a greater rate than the I.sup.2 R heat generated in the pipe and what is required is a reduction in the cable resistance which can only be accomplished by adding cables in parallel.
The effect of this misuse of the thawing apparatus is that the cables become excessively hot giving rise to a tendency for the cable connections to melt and the connections to come off. Moreover, since this increased heat is at the expense of the heat applied to the pipe it can take an unacceptably long time to thaw out a pipe and, in fact, where the cables are long or the pipe diameter is large, the pipe may never thaw. Further, because of the proportion of the I.sup.2 R heat lost in the cables, a very high capacity generator is required. Additionally, because of the length of time of application of heat there is a danger of melting the lead in the joints in the pipe and of burning valve gaskets and packings.
Another effect of excessive current in the cables is a large variation in the voltage applied at the pipe between load and no-load. The result is that should the pipe circuit become open the voltage immediately rises substantially, thereby forcing an excessive current over the neutral of the electrical distribution system, which system is parallel with the water system and is not designed or intended to carry excessive current. This excessive current heats up the ground wires to the point where they can ignite combustible materials or even to the point where they fuse and create an arc.
It is an object of the present invention to obviate or mitigate the above defects.